WASHINGTON, May 9 (Reuters) - The United States warned on Sunday that Israel and the Palestinians will be held accountable if they do anything that seriously undermines the peace effort.

Assistant Secretary of State Philip Crowley said in a statement that US special envoy George Mitchell will return to the Middle East next week to try to foster the indirect peace talks and urged both sides to keep up the effort.

As both parties know, if either takes significant actions during the proximity talks that we judge would seriously undermine trust, we will respond to hold them accountable and ensure that negotiations continue, Crowley said.

He did not specify what sort of actions could undermine the process but the United States has long-standing concerns about Israeli settlement construction and Palestinian violence.

Mitchell has not made any public comments since the Palestine Liberation Organization approved four months of indirect talks Saturday.

The discussions, which opened on Sunday, are the first Middle East peace negotiations in 18 months.

Crowley said the United States has received commitments from both sides and made assurances to both in order to keep the talks going but gave no details. The full scope of these discussions will remain private, he said.

Crowley said Mitchell wanted progress in the talks so that the two sides can move to direct negotiations that will result in a two-state solution to the Israeli-Palestinian conflict.